Air treatment systems, such as air conditioning and cooling systems, preferably include some means for controlling air humidity. Air humidity can be just as important to personal comfort as air temperature, especially in sunny regions close to open water. Also, air humidity control can be important for the protection of sensitive electronics, ancient art treasures etc.
Whereas raising air humidity is a relatively straightforward operation, it takes a lot more to bring it down for the purposes described above.
Various methods exist for bringing down air humidity. The spraying of certain chemicals is an non-repeatable and comparatively expensive method with many additional disadvantages. In general, a repeatable method is preferred, in the sense that no refuelling is needed, other than a possible energy supply. Air cooling means may indeed effect a repeatable drying of air, but since that is inevitably coupled to the cooling thereof, it will in itself not allow enough flexibility and fine-adjustment to meet the needs of most applications described above. On top of that, it makes for a relatively inefficient and therefore expensive air drying process.
More sophisticated desiccant means have therefore been developed, that are often but not always used as part of an air cooling system. Best described are the so-called desiccant wheels, that can for instance be found in U.S. Pat. No. 5,353,606, U.S. Pat. No. 4,719,761, U.S. Pat. No. 4,594,860, U.S. Pat. No. 3,488,971, U.S. Pat. No. 3,247,679 and U.S. Pat. No. 3,009,684, all deemed to be incorporated by reference. The thermodynamic process involved in operating these desiccant wheels are also described therein. Examples of the active chemical compound used in such desiccant wheels are silica-gel and lithium chloride.
Most air humidity controlling systems need some form of control over the level of activity employed at any given time. When drawing outside air, the humidity and temperature thereof vary and influence the end result. Also, the end result itself may have to be adjusted from time to time, for instance depending on personal preference in the case of comfort control. All forms of level control in an air humidity controlling system require measurement of air humidity, be it of the end result, be it at some intermediate stage, be it of the outside air.
Air humidity measurement is usually performed by comparing the air humidity in two different areas that indicate the air humidity before and after treatment with the desiccant means. The comparison in turn gives an indication of the preferred level of activity. For instance, one area could be somewhere within a duct for transporting outside air to the inside of a building, and the other area in another duct serving the opposite purpose. Or one area could be the inside of the building itself, so that the end result of the air humidity control is being measured, and the other measurement could then possibly be performed outside, in order to give the system an indication of the difference that should be bridged. In theory, one humidity sensor placed in between the two different areas of measurement may do the job. However, the most practical solutions so far make use of two separate humidity sensors, each located with either of the two measuring areas.